The processes for converting hydrocarbons at high temperatures, such as for example, steam-cracking, catalytic cracking or deep catalytic cracking to produce relatively high yields of unsaturated hydrocarbons, such as, for example, ethylene, propylene, and the butenes, are well known in the art. See, for example, Hallee et al., U.S. Pat. No. 3,407,789; Woebcke, U.S. Pat. No. 3,820,955, DiNicolantonio, U.S. Pat. No. 4,499,055; Gartside et al., U.S. Pat. No. 4,814,067; Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Li et al., U.S. Pat. No. 4,980,053; and Yongqing et al., U.S. Pat. No. 5,326,465.
It is also well known in the art that these mono-olefinic compounds are extremely useful in the formation of a wide variety of petrochemicals. For example, these compounds can be used in the formation of polyethylene, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylonitrile, methyl tertiary butyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber.
Besides the mono-olefins contained in the cracked gases, the gases typically contain a large amount of other components such as diolefins, hydrogen, carbon monoxide and paraffins. It is highly desirable to separate the mono-olefins into relatively high purity streams of the individual mono-olefinic components in order to facilitate downstream processing. To this end a number of processes have been developed to make the necessary separations to achieve the high purity mono-olefinic components.
An especially significant process for recovering olefins from cracked gases is a selective chemical absorption process described in Barchas et al., U.S. Pat. No. 5,859,304. In the Barchas et al. '304 patent, there is suggested the desirability for removing at least substantially all acetylenes and dienes from the depropanized cracked gas prior to the chemical absorption of the olefins. Removal of these acetylene and diene contaminants is beneficial because the acetylene and methyl acetylene will react with the absorbent solution to form potentially hazardous acetylides, and propadiene will be absorbed along with the olefins by the chemical absorbent thereby eventually contaminating the propylene product.
In Barchas et al. '304, a "deep" front end selective hydrogenation process is taught to effectuate the removal of at least substantially all of the acetylenes and dienes from the depropanized cracked gas prior to demethanization. The deep front end selective hydrogenation process disclosed in Barchas et al. '304 operates in the vapor phase and uses multiple catalysts beds, typically three with cooling between the beds. However, due to the severe conditions required in the last bed to hydrogenate the last traces of propadiene, it has been found that the process concomitantly hydrogenates some of the ethylene into ethane, typically around about 3%. The hydrogenation of ethylene into ethane increases the ethane recycle to the cracking furnaces, which results in increased feedstock consumption, larger equipment and increased processing costs. There is also a potential for the olefins to react uncontrollably with the large excess of hydrogen present in the cracked gas (a so-called runaway reaction). Additionally, the deep hydrogenation process of Barchas et al. '304 also has the disadvantage of running at relatively low space velocities (i.e., high gas residence times corresponding to larger catalyst volumes) in order to achieve the deep hydrogenation required.
Therefore, although the Barchas et al. '304 process has proved very beneficial to the art of recovering olefins from cracked gases, it would be highly desirable to provide an improved deep selective hydrogenation process which overcomes the aforementioned drawbacks.